A large scale gate provided against high tide water, tsunami etc. is well known.
The gate of Patent Document 1 is a flap gate, which includes a gate leaf (torsion structure) that has a thin wall closed cross-section, and axle type supports supporting the gate leaf. The gate leaf is supported by a foundation ground via the axle type supports and rotates around the axles.
FIG. 1A and FIG. 1B show an example of an axle type support for a flap gate, where FIG. 1A shows its side elevation view and FIG. 1B shows a cross-section cut along line A-A of FIG. 1A.
Reference numeral 6 denotes a gate leaf (solid line, in a closed position), 7 denotes the gate leaf (dotted line, in an open position), 8 denotes a bottom support, 9 denotes a rotation axle, and 10 denotes a bracket.
The gate leaves 6 and 7 are fixed by welding etc. to the bracket 10 that is connected to the rotation axle 9. The bottom support 8 is sustained by a foundation ground.
When the gate is not in use, the gate leaf (in its open position) 7 is stored horizontally underwater as the dotted line shows. When in use, the gate leaf (in its open position) 7 rotates around the rotation axle 9, rises up, and moves to the position of the gate leaf (in its closed position) 6 of the solid line.
FIG. 2A and FIG. 2B explain a difference in characteristics of deformation between torsion and bending type structures. FIG. 2A shows the bending type structure and FIG. 2B shows the torsion structure, where L denotes span length.
A characteristic in the deformation of the bending type structure is the parallel displacement of its cross-section while that of the torsion structure is the in-plane rotation of its cross-section. The rotation center of the cross-section is the axle type support that restricts the displacement of the cross-section. The torsion structure is distinguished from the bending type structure by whether or not there is a restriction point on the cross-section.
Structural characteristics of both of the structural types are remarkably different when their cross-section is a thin wall closed cross-section. In short, the torsion structure is characterized by (1) the thin wall closed cross-section and (2) the cross-sectional restriction.
The torsion structure resists a load by square of its closed cross-sectional area while the bending type structure and the axial type structure resist by the cross-sectional secondary moment and axial rigidity of their members, respectively.
A load applied to the torsion structure is transmitted to a sectional restriction point, and a torsion moment composed by the load and the reaction force at the restriction point is transmitted to the support span terminal of the structure due to a sectional torsion rigidity while the loads applied to the bending and axial type structures are directly transmitted to their support span terminals due to a sectional shearing rigidity and an axial rigidity, respectively.
The bending type and axial type structures are 3-dimensional structures whereas the torsion structure may be classified as 2.5-dimensional structure.
The torsion structure has various advantages due to the structural differences described above, and these advantages become more remarkable as the structure support span gets longer. In the case of a 400 m span class super large gate, for instance, its steel weight will be ½ to ⅓ or less of other structural types. The lower gate weight results in lower construction costs.